A power system may include distributed power sources (e.g., distributed generators, battery banks, and/or renewable resources like solar panels or wind turbines to provide power supply to a grid (e.g., a microgrid having local loads and/or a utility grid). The power system may include a power converter, such as a power inverter, for converting power between a power source and a grid. Such power conversion may include AC/DC, DC/DC, AC/AC and DC/AC.
A micro-grid system can include a variety of interconnected distributed energy resources (e.g., power generators and energy storage units) and loads. The micro-grid system may be coupled to the main utility grid through switches such as circuit breakers and/or contactors. In the event that micro-grid system is connected to the main utility grid, the main utility grid may supply power to the local loads of the micro-grid system. The main utility grid itself may power the local loads, or the main utility grid may be used in combination with the power sources of the micro-grid to power the local loads.
A controller comprising hardware and software systems may be employed to control and manage the micro-grid system. Furthermore, the controller is able to control the on and off state of the switches so that the micro grid system can be connected to or disconnected from the main grid accordingly. The grid connected operation of the micro-grid system is commonly referred to as “grid tied” mode, whereas the grid disconnected operation is commonly referred to as “islanded” or “stand alone” mode. A micro-grid system in grid tied mode should be capable of disconnected from the main grid and transitioning to islanded mode in the case of a grid event in which abnormal operation conditions, such as a power outage, occur at the main utility grid.
When the micro-grid includes a battery bank, a battery energy storage system may be used to provide power to, or to receive power from, the micro-grid. The battery energy storage system can be used as an energy storage unit in a smart grid system. Renewable energy sources such as photovoltaic/solar panels and wind turbines are intermittent sources subject to unpredictable and inconvenient weather patterns. The generation source rarely matches the load needs; and therefore, it is desirable to provide energy storage units. The use of energy storage units, which can both store and supply power, allows the micro-grid system to provide reliable and stable power to local loads.
The energy storage units can also store excess energy from the renewable sources (and potentially the grid). For example, renewable energy generation may exceed load demand of the micro-grid. Without energy storage capability, the extra generation is lost. If energy storage units are employed in the micro-grid, the extra generation can be captured by storing it in the batteries. The energy storage units can then supply this power to local loads and even the main utility grid where appropriate.
In a power system such as the battery energy storage system described above, the power source or storage unit is not constantly providing power. For example, in the case of a battery bank providing grid stability services such as automatic voltage response, the batteries may be neither discharging nor charging. Power systems, such as the battery energy storage system discussed above, have an “off” and an “on” state. During the “off” state, the switches are open and the inverter is not synchronized to the grid. Thus, the “off” state can be used to conserve power when the battery bank is neither charging nor discharging. However, the switches are mechanical or electromechanical switches such as contactors. Thus, when the system needs to operate (e.g., power needs to be supplied to or from the battery), the startup time is limited by the closing time of these mechanical switches, which may be up to several hundred milliseconds or several utility voltage cycles. A delay of this magnitude is undesirable to a user of the power system.